qa: add 276 guard for QT env var, causing some hangs

Was fine on some platforms, triggering QA watchdog on others
so increase the level of protection for those (older) cases.

qa/276

@@ -25,8 +25,15 @@ trap "rm -rf $tmp $tmp.*; _clean_display; exit \$status" 0 1 2 3 15
 if [ -z "$DISPLAY" ]
 then
     export DISPLAY=$PCPQA_CLOSE_X_SERVER
+    PCP_STDERR=$tmp.stderr QT_QPA_PLATFORM=offscreen pmchart --help >/dev/null 2>&1
+    if [ -s $tmp.stderr ]
+    then
+        if grep 'Usage: pmchart' $tmp.stderr >/dev/null
+	then
+            export QT_QPA_PLATFORM=offscreen
+	fi
+    fi
 fi
-export QT_QPA_PLATFORM=offscreen
 
 src/grind-tools -v -a archives/fcsw_indom fcsw.port.bytes_out 2>>$seq.full \
 | _filter_views